Photocell



Sept. 7, 1948.

R. J. CASHMAN PHOTOCELL Filed Feb. 9, 1944 5 Sheets-Sheet 1 Sept. 7, 1948. R, 1 CASHMAN 2,448,517

PHOTOCELL Filed Feb. 9, 1944 5 Sheets-Sheet 2 Sept. 7, 1948. R. J..cAsHMAN PEOTOGEIL 5 Sheets-Sheet 3 Filed Feb. 9, 1944 Sept 7, 1948- R.'J. cAsHMAN 2,448,517

PHOTOCELL Filed Feb. 9, 1944 5 Sheets-Sheet 4 Sept. 7, 1948. R. J. CASHMAN 2,448,517

P'HoTocELL Filed Feb. 9. 1944 5 Sheets-Sheet 5 k. (all @924127217649 :nwo/bvs ZJ'r/Ocmmd /JZ 5 'll/j() ..5 .6 ,8.910.21/12131( o f z J 4 5 o' 7 859/0 a [QW .771m in' Mzafes iatented Sept. 7, 1948 UNITED STATES PATNT Torrice- .l 2.44am y PBOTOCELL n Application February s. 1944, serial Nn. 521,594

(c1. ce1-c3 21 Claims.

This invention relates generally to photo-electric or photo-sensitive cells, of which there are three types: photo-resistive or photo-conductive,

photo-emissive, and photovoitaic. Selenium.

cuit is functionally related to the'int'ensity of.

the light falling upon the cell. The photo-'voltaic cell converts light energy into electrical energy directly without the assistance of an external electromotive force.

Although this invention has certain applica-- bility to all three types of photo-sensitive cells, the more speciilc aspects of the invention relate to the photo-resistive or photo-conductive type of cell, and for the purpose oi' this disclosure the invention will be described with particular reference to that type of cell.

It is known that thallium sulilde, or more particularly thallous sulilde has unusual pho'toelectric properties and is particularly responsive to radiations in the infra-red region vof the spectrum, but heretofore photocells employing thallous sulde have not been reliable in operation because of their inherent instability. I have discovered how to produce thallous sulilde photocells which are stable over long periods of time and which have other unusual desirable operating characteristics.

Some of the principal objects of the invention, therefore, are: To produce a photocell of the photo-conductive variety which is particularly sensitive to radiations in the infra-red region of the spectrum; which is inherently stable and docs not lose its sensitivity when exposed to strong light intensities; which has a dark resistance that is sumciently low that it may be used eiIection is read in conjunction with the accompanying drawings, in which tively with thermionic amplifying apparatus and still have the required sensitivity to radiations in the infra-.red region of the spectrum, particularly between 8,000 and 13,000 angstroms;which has an extremely low noise factor as compared with other cells of this type. and which can be -produced economically in quantity production.

Fig. 1 is a vertical sectional view showing one form of photocell employing my invention;

Fig. 2 is a horizontal sectional view taken on the line 2-2 of Fig. 1; Y

Fig. 3 is a fragmentary sectional view taken on the line 3 3 of Fig. 1, showing the manner in which the photo-sensitive element is braced against the wall of the envelope which encloses it:

Fig. 4 is a vertical sectional view showing another form of my invention in which the photosensitive element is disposed transversely to the longitudinal axis of the tube;

Fig. 5 is ahorizontal sectional view taken on the line 5 5 of Fig. 4;

Fig. 6 is a plan view showing another way in which the photo-sensitive element may be supported within the photocell tube;

Fig. 7 is an elevational view of the form of the invention shown in Fig. 6;

Fig. 8 is a vertical sectional view showing what is now considered to be a preferred form of the invention;

Fig. 9 is a horizontal sectional view taken on the line 8-9' oi Fig. 8;

Fig. 10 is an enlarged fragmentary view showing the manner in which a portion of the interior wall of the photocell envelope may be roughened or ground to receive the grid lines;

Fig. 11 is a vertical sectional View showing another form of the invention;

Fig. 12 is a horizontal sectional view taken on the line i2--I2 of Fig. 11;

Fig. 13 is a schematic drawing showing the apparatus used in making the photocells;

l Fig. 14 is a plan view showing the ruling apparatus used for applying the grid in the form of the invention shown in Figs. 8 and 9;

Fig. 15 is an elevational view of the same;

Figs. 16 and 17 are vertical sectional views taken on the line IG-IS and i'l--Il of Figs. 14 and 15, respectively, and

Figures 18 to 20 are graphs illustrating diagrammatically the physical characteristics of cells in accordance with the present invention as compared with those of the prior art.

At the outset, it should be understood that the description of certain preferred forms of the invention is for the purpose of disclosure only and should not be construed as limiting the appended claims, except as may be required by the prior art.

It should also be understood that I do not wish' to be limited to the theory which may be exand the method of making cells of this type which I desire to protect by Letters Patent.

Pnrsrclu. Sraocruaa or ma Pnorocaaas In th'e form of the invention shown in Figs. 1-9, inclusive. the photocell comprises an envelope 2l, preferably made of a high melting point borosilicate glass such as Nonex or Pyrex glass, both of which readily transmit in the infra-red region of the spectrum, the top of the envelope being closed at 2| and the bottom of th'e envelope having a re-entrant portion 22 through which' electrodes 29 and 24, preferably of tungsten, are pinch-sealed. A small glass tube 2l is sealed into the reentrant portion 22 of the envelope to permit the envelope to be suitably evacuated and, if desired, gas illled. The tube 251s sealed of! at 29 after the cell has been appropriately processed, as hereinafterdescribed.

Mounted upon th'e electrodes 23 and 24 is a photo-sensitive element generally designated 21 which comprises a glass plate 29, also preferably of Nonex or Pyrex glass, into the two sides of which tungsten conductors 29 and 39 are sealed. The plate may be at as shown, or may be curved to conform more or less with the adjacent envelope wall. The glass plate is then beveled oil' by grinding, as shown at 3l, to expose th'e length of the conductors 29 and 30; and these conductors also project above and below the glass plate 29, as best shown in Fig. 1. The projection of the conductors 29 and 99 below the plate 29 forms a convenient means by which the photo-sensitive element 21 may be mounted upon the electrodes 29 and 24, and th'e supporting rods of nickel 32 and 33 are spot-welded to the conductors 29 and 30 and the electrodes 23 and 24, respectively, to support the photo-sensitive element in piace.

The upper projecting portions of the conductors 29 and 90 have small spring clips 94, preferably of molybdenum, spot-welded thereto for the purpose of furnishing additional support to the photo-sensitive element 21 within th'e envelope (see Fig. 3).

Due to the relatively high resistivity of thallous sulilde, it is necessary, for practical reasons, to employ a grid on the photo-sensitive element in order to hold down the dark resistance of the cell as a whole. The grid may be formed in various ways, but it consists essentially of a plurality of parallel, relatively ilne conducting lines with' all even numbered lines connected to one terminal of the photo-sensitive element and all odd numbered lines connected to the other terminal of the element. The two sets of intercalated conducting lines in eil'ect break up the photo-sensitive surface into a. plurality of parallel connected resistances. so that the overall resistance oi' the cell is kept within the desired limits.

Referring now to Fig. Lit will be seen that the plate 29 has a grid applied to one of its faces, `and this grid comprisesa plurality of odd numbered, relatively fine conducting lines 35, the ends of whichare connected by a relatively broad conducting line 38, and a plurality of even numbered, relatively ne conducting lines 31 with their ends connected at the opposite margin of the plate by a relatively broad conducting line 2l.

The conducting lines may be graphite applied bylead pencilox" tedfrcm an aqueous colloidal graphite suspension. The lines may also be formed of gold. platinum, or any other suitable conducting material which lends itself to application in relatively ilne lines. The relatively broad conducting lines Il and 39 applied to the side margins of the plate 2l are in intimate electrical contact with the exposed portions of theeonductors 29 and Il respectively, which. in turn. are connected through the nickel carrier rods 32 and 93 and the electrodes 2l and 24 with external leads 89 and 4l.

Further details of the grid and its method of application will be described latex on with particular reference to its bearing 90u certain operating characteristics of the Dhotocell, but. for the present, it will be sumcient to understand that the photo-sensiive material. whether it be th'allous sulnde or some other material, is applied over the grid to complete the photo-sensitive element 21, as indicated at 4i.

In the form of the invention shown in' Fig. 4, the photo-sensitive element, generally designated 49, is mountedin a plane that is normal to the axis of the photocell. The element consists of a glass plate 4l. preferably of Nonex or Pyrex. through which at least two. and preferably four, tungsten conductors 41 are sealed. The upper ends of the conductors 41 are ground oil ilush with the top of the glass plate 46, and the grid is applied to the top surface of the plate in much the same manner as described with reference to the form of th'e invention shown in Figs. l-3, inclusive. The relatively ilne conducting lines comprise odd numbered lines 49 connected at their ends by a relatively broad conducting line 49, which is in intimate electrical contact with the exposed ends of the conductors 41' sealed through the right side (Fig. 5) of the plate 44. Th'e even numbered conducting lines 50 of the grid are connected at their ends by a relatively broad conducting line 5i, which likewise is in intimate contact with the exposed ends of the conductors 41, sealed through the left side (Fig. 5) of the plate 49. As before, the photo-sensitive material is then deposited upon the surface of the grid, as will hereinafter be described.

The photo-sensitive element 45 is supported on the electrodes 52 and 53 by nickel carrier rods 54, which are spot-welded to the lower ends of the tungsten conductors 41 and the upper projecting ends of the electrodes 52 and 53. The lower ends of th'e electrodes 52 and il are connected to external leads 59 and 59, respectively.

In both forms of the invention-i. e., the form shown in Figs. 1-3. inclusive, and the form shown in Figs. 4 and 5-the cell envelope is preferably based to protect the cell and provide a convenient means for mounting the cell in an operating cir' cuit. It is important, however, for the leads to be well insulated from each other due to the high resistance characteristics of the cell, and to this end the base 58 may be formed, as in Fig. l, of polystyrene or Isolantite. both of which are nonhygroscopic and of relatively high resistance. When a metal base is used such as 59 (Fig. 4), a bottom plate 90 is employed to provide the dcsirable high resistance characteristics. The bases are provided with base pins Si, as is common.

A slightly modiiled mounting for the photosensitive element is shown in Figs. 6 and 7, and in this form of the invention the photo-sensitive element is made in exactly the same manner as the element 21 (Figs. 1-3, inclusive), but the element is .mounted in a plane normal to the axis of the cell. In this instance, the projecting ends of the conductors 23 and 3| are spot-welded to nickel carrier rods 32, which, in turn, are spot-welded to the electrodes 23 and 2l. The carrier rods l! are U-shaped, as best shown in Fig. 6, and constitute a convenient means for mounting the photosensitive element on the supporting electrodes.

The form of the invention shown in Figs. 8 and 9 is at present preferred because of the simplicity of its structure and because in some respects it has better Hoperating characteristics.

The photocell comprises an envelope I3 closed at its upper end and having tungsten electrodes N and 35 sealed through the bottom wall of the envelope. As before, the envelope is preferably made of a high melting point glass, such as Nonex or Pyrex. An exhaust tube 68 is also sealed into the bottom wall of the envelope to enable the cell to be evacuated and lprocessed in a manner later to be described. After the cell has been processed, the exhaust tube 66 is sealed off in a conventional manner.

In this instance, the photo-sensitive element is formed on the inside wall of the envelope itself, and preferably extends around one half of the envelope periphery. The grid is applied to the inside wall of the envelope in any convenient manner, preferably by ruling with an aqueous colloidal graphite suspension. As before, the odd numbered grid lines 61 have their ends connected by a relatively broad conducting line 63, which extends vertically along the interior side wall 0f the envelope and then horizontally over the exposed and ground off inner face of the electrode 65. Likewise, the even numbered grid lines 88 are connected at their ends by a vertical, relatively broad conducting line 10, which at its lower end extends laterally along the bottom wall of the cell envelope over the exposed and ground off end of the electrode 6I. The photo-sensitive material 4i is then deposited over the grid and activated to complete the photo-sensitive element.

When colloidal graphite ls used for making the grid line 61, the inside surface of the envelope wall may be left in its normal glazed finish; but, when a lead pencil is used for forming the grid, it is necessary to roughen the portion of the envelope which is to receive the grid, as indicated in Fig. 10 at 1i.

One advantage of the type of photocell shown in Figs. 8, 9, and 10 is that the cell will respond to radiations from any direction within a plane normal to the axis of the cell. Although the cell shown in Fig. 1 will respond to radiations entering the cell from either side of the plate 23, it will not respond to radiations which enter the cell along the lateral side edges of the photo-sensitive element 21. In the cell shown in Figs. 8-10, however, the radiations may come from any direction within a plane normal to the axis of the cell, and this 360 degree responsiveness of -the cell makes it desirable for certain types of applications. Of course, the radiations need not necessarily be within a plane normal to the axis of the cell -provided there are components which lie in such plane.

The photocell shown in Figs. 11 and 12 is of the end-on variety with its photo-sensitive element 12 lying in a plane normal to the axis of the cell. It is formed on the closed and flattened end 13 of a Pyrex or Nonex tube 14. Tungsten electrodes 15 and 18 are sealed angularly through opposed margins of the wall 13, and the ends of the electrodes projecting into the encasing envelope shell 11 are ground off, after which a grid consisting of broad terminal lines I8 and 13, connecting the ends of even numbered grid lines and odd numbered grid lines 3|, respectively, is applied to the interior surface of the wall 13. Y

The lower end of the tube 1l is flared outwardly, as indicated at 32, where it is sealed to the encasing envelope 11. An exhaust tube 83 is sealed into the reentrant portion ofthe cell for exhausting and other processing of the cell. After processing. it is sealed oif, as indicated at 84.

The photocells shown in Figs. 8 and 11 are also appropriately based as indicated at 85 and 86, respectively, and the lead wires welded to the electrodes of the cells extend through the base pins, to the bottom of which they are soldered, as indicated at 81.

PREPARATION or THALLoUs SULrrnE Thallous sulfide may be prepared in various ways, although there are only two ways which are fundamentally different. The first method starts with commercial thallium sticks which are heavily oxidized. These are melted down in a glass vacuum system, during which time the oxide breaks down (about 300 degrees centigrade) The molten material is carefully poured into aconnecting chamber, leaving behind a certain amount of slag material which sticks to the glass. This process is repeated by pouring the thallium into another -connecting chamber following the first one, and nally pouring it into a long connecting tube, where it is allowed to freeze. During these operations, the pressure is maintained at a `few microns. 'Ihe tube containing the silvery white thallium is then removed from the vacuum system and a known portion is then introduced into one side arm of another chamber (a tube with two side arms). A second side arm contains sulphur previously fused from chemically pure flowers of sulphur. The mass of sulphur employed is dependent upon the mass of thallium selected, and is determined in the following way: The thallium sulfide molecule (TlzS) contains by weight 92.73% thallium. The mass of sulphur placed in the side arm is such that there is a slight excess of thallium, actually about 13 parts thallium to one of sulphur by weight. This is done to assure that the end product will be T12S rather than some intermediate compound or product between TlzS and TlzSz, as would be the case were an excess of sulphur present. vThe chamber is evacuated and the thallium is again melted down. Any oxide which has formed breaks down. The sulphur is gently heated to drive off air and Water vapor, and. finally the chamber is sealed olf. .The melted thallium is allowed to run into the main chamber, and the sulphur is slowly vaporized from the side tube. The melted thallium reacts with the sulphur vapor very rapidly and forms a compound `of higher melting point. VThe temperature in the main tube must be raised to about 460 degrees centigrade before the new compound melts. Finally, all lof the sulphur is evaporated and taken up by the free thallium. After several minutes, it will be observed that there is la small amount of liquid at the bottom which has more of a metallic appearance than the remaining liquid above. Chemical analysis shows that the heavier liquid is practically pure thallium while the lighter liquid is TlzS. After cooling to room temperature, a portion of the solidified 'IlzS is crushed,v

in a mortar'to a fine powder and stored in an evacuated container for future use in the cells.

The second method of preparation starts with 7 commercial thalllum nitrate or sulfate. About ten grams is dissolved in twice distilled water and recrystallized from solution three times. This is dissolved in about 500 cc. of twice distilled water. Precipitation is carried out with hydrogen sulfide prepared from iron sulfide and dilute hydrochloric acid. 'I'he HaS is passed over moist pumice and then washed with distilled water prior to use. Two or three cc.'l of ammonium hydroxide are added to the s tion to keep it alkaline during the precipitat n of the sulfide. The HzS is bubbled throug the solution until precipitation stops. Th precipitate is filtered on a sintered glass cruel/lille connected to a. water aspirator, then washed wit HzS water, and finally distilled water. It is stored in an evacuated desiccator over P205 to dry. `\Part of the black TlzS powder is then fused in vacuum in a hard glass fusion tube. When cold, the bead is crushed in a mortar and stored in vacuum prior to use.

In some practices of my, invention, I ind it desirable to take steps to remove such metallic impurities as lead, antimony, and copper. This is done in any well known manner, but I prefer to accomplish this result by using HzS to precipitate out these impurities from an acid solution of the nitrate or sulfate. The precipitate is then filtered out and the solution made alkaline, after which the process continues as heret-ofore described.

METHOD or CoNsrRUcTING AND PROCESSING A PHo'rocELL In order to produce stable, highly sensitive photocells, great care must be exercised in the construction and processing of the cell. While it is possible to obtain stable cells which have a higher degree of sensitivity than those heretofore known without difficulty by the practice of my invention, it is only with the exercise of great care and precision in. constructing and processing the photocell that one is able to produce a stable photocell of extremely high sensitivity. Hence, it should be understood that for many applications of my invention, particularly those in which super-sensitive cells are not required, not all steps of the hereinafter described fabrication of the cell need be slavishly followed; and, even when supersensitive cells are desired, some modification of the processing is possible.

To illustrate a preferred method of making a photocell, let us consider, first, the step by step process by which the cell shown in Figs. 8 and 9 is constructed and processed.

I first start with a piece of Nonex or Pyrex tubing, about 3 inches long and about 30 millimeters in diameter, and the botto/m/ end is closed and flattened. The exhaust/tube 66 is sealed into place. as are also the electrodes 64 and 65, after first being beaded with glass and ground ofi' iiush at their upp/er, ends. If the grid is to be applied with a lead'pencil, the inside surface of the tube which is to receive the grid is at this time ground with 600 mesh carborundum. 'I'he envelope is then thoroughly cleaned by scrubbing, and then washed repeatedly with hot water. The exposed ends of the electrodes 64 and 65 are then cleaned electrolytically, using a sodium hydroxide solution. After rinsing the tube out with hot water, a cleaning solution composed of sulphuric acid and potassium dichromate is used. 'I'his is flushed out with hot water again, and the envelope is then dropped into a 1% solution of hydrofluoric acid, where it is left for about seconds, and then thoroughly washed several times with boiling distilled water. It is then placed in an oven'to dry. The envelope is now ready to receive the grid.

As stated before, if the grid is to be applied by pencil, it must be upon a ground surface. as indicated at 1I in Fig. 10, but if the grid lines are to be deposited from a colloidal graphite suspension. or from some other metallic colloidal suspension, or if the grid lines are to be applied by evaporatlng the metal onto the wall through a stencil. the interior surface of the envelope may be left in its natural state. I have found that for the best results a commercial product known as Aquadag. manufactured by the Atcheson Colloidal Graphite Company, of Port Huron, Michigan. v"should be used, this product being an aqueou colloidal graphite suspension. Among other things. it has the property of being chemically inert to thallium sulude, which makes it preferable to other conducting materials, such as gold or platinum, which appear to alloy with thallium sulfide to some extent. Hcwever. gold, platinum. or the like may be used if covered with Aquadag, or if the gold, platinum, or similar conducting material is applied in sufficiently heavy coatings, as, for example, by evaporation onto the glass wall through a stencil. a satisfactory cell may be obtained without an over coating of Aquadag.

When the grid is applied to the cell by ruling. as distinguished from evaporation through a stencil, it is convenient to use a device such as shown in Figs, 14-17. inclusive. 'I'his device consists of a base having a semi-cylindrical bearing li at one end adapted to receive a relatively heavy cylindrical bearing 92. which is connected through a hinged joint 93 with an arm 94 carrying a bow pen or other marker 95 at its extreme end. The cylinder 92 is provided with laterally extending trunnion pins 96, which are adapted to snugly seat within notches 91 provided along the upper surface of the semi-cylindrical bearing li. The notches are spaced apart a distance corresponding to the distance desired between grid lines, which, preferably, is about one-fifteenth of an inch.

The cell which is to receive the grid ruling is clamped to the base 90 by adjustable clamps ll, which firmly hold the envelope 99 in an appropriate lposition to receive the end of the arm Il and the bow pen 95. The lines are ruled onto the interior surface of the envelope 99 by rotating the arm 94 and the associated bearing 92. and a handle |00 is provided for convenience of manipulation. After a grid line has been ruled, the bearing 92 is moved rearwardly a notch and the next line ruled, and the procedure continues until the desired number of lines has been ruled to produce a cell of the desired resistance values and target area.

In using Aquadag, or any similar conducting material. the suspension should be as concentrated as possible without interfering with proper flowage of the material through the bow pen DI.

After the grid lines 6l and 69 have been ruled onto the envelope, the conducting lines il and 'II (Fig. 8) are applied to the envelope. These lines are also preferably of Aquadag, and they extend down the sides of the envelope and laterally over the adjacent electrode. These lines are preferably applied by brush.

It should be understood that the Aquadag may be satisfactorily applied to the interior surface of the envelope only after the envelope has been thoroughly cleaned, as. for example. by the cleaning steps heretofore set forth.

After the grid has been applied to the envelope and electrically connected to the electrodes. the

eertained at any given time.

cell 1s ready for a preliminary baking out. The tubes are heated to 500 degrees centigrade fifteen to twenty minutes to remove any organic material presentin the Aquadag, and then the upper end of the envelope is sealed oil'. The envelope is then connected to a vacuum and p system such as shownin Fig. 13, the connection being made through the exhaust stem Il.

As shown in Fig. 13. the apparatus used in this stage of the process consists of a glass manifold III having a plurality of branches |02 adapted to be connected to the exhaust stems Il of the cells being processed. The manifold III is con- 'nected through liquid air traps Ill and I to a mercury diffusion pump, generally designated III, which may be of conventional design and capable of producing a vacuum of 10 millimeters when connected in series with a mechanical vacuum pump Ill. A resistance oven |01, closed at |08, can bc slipped over the manifold I Il and the cells that are being processed until it engages the cover IDI, and a thermometer III projects through the cover I Il so that the temperature within the oven may be readily as- The oven is connected by 'leads to any suitable source of electrical power.

After the cells have been connected to the manifold Ill, the oven |01 is moved into pla-ce and the cells are heated to 550 degrees centigrade with the pumps |05 and I" operating continuously at all times. After the cells are thoroughly baked out in this manner, they are ready to receive the thallium sulflde powder which has been prepared in the manner heretofore described. The purpose of this baking out is to remove from the interior of the envelope all impurities which may be contained therein, including, particulary, vapor and gases occluded in the glass envelope.

The cells are then removed fromthe manifold by opening the exhaust stems 66, and approximately l5 milligrams of thallous sulfide, prepared as heretofore described, is introduced into each envelope through the exhaust stem 65, preferably while the envelope is being held in a horizontal position with the grid positioned above. The thallous sulfide is shaken so that it is well distributed along the length of the cell beneath the grid, and the exhaust tube 68 of each cell is again connected to the manifold |I. After the cells are sealed on and time has been allowed for the pumps and |06 to exhaust the cells, the oven is again slipped in place and the cells are heated to a temperature of about 425 degrees centigrade, keeping the pumps at all times in full operation. The oven is then removed and the cells allowed to cool in air, still being continuously evacuated. When cool, the thallium sulfide is evaporated with a gas air burner, and the thallium sulfide condenses over the grid. The cells are then allowed to cool to room temperature, and the cells are then ready to be activated. At this stage, the cells llave very little sensitivity to radiant energy.

An alternate method for forming the thallium sulfide deposit on the grid is by introducing thallium and sulphur into the cell separately and causing the reaction to take place in situ. A given amount of thallium is evaporated in the evacuated cell and deposited on the grid in a thin layer in the same manner as TlzS, and then a limited amount of sulphur vapor, either in a pure, uncombined state or a combined state such as HzS, is permitted to come in contact `important one, because it is in this stage that l0 with the thallium while the cell is heated to a temperature of about 280 C. The reaction is allowed to take place until the electrical resistance of the cell reaches a maximum, at which time the sulphur vapor is cut oil. The activation stage of the process is a ver'y the desired operatingcharacteristics of the cell are attained. It is a peculiar DIOPerty of thallous sulnde that it reaches its maximum sensitivity `to radiant energy when oxidized and heat treated to some definite limited degree. Any oxidation or heat treatment which falls short of the desired amount. or exceeds such value, results in a cell which will not have maximum sensitivity. i

' There are various ways in which photo-sensitization may be eilected. 'I'he quantity of oxygen permitted to come in contact with the thallous sulfide, the temperature to which the material is raised, and the 'time permitted for oxidation are all factors in determining the extent4 -the example is purely by way of illustration and that the procedure may be varied, giving appropriate consideration to the three factors mentioned above which affect the extent of accell having above average sensitivity-it is sometimes desirable to determine empirically the exact conditions for activation consistent with the knowledge that limited oxidation is required and the further fact that there are sometimes variations in Athe physical properties of different -batches of thallous sulfide even though prepared in apparently identical ways'.

An example of a procedure for activation to produce cells of average sensitivity is as follows: After the thallous sulfide has been deposited on the grid by evaporation, as heretofore described, and the cell has been continuously kept under high vacuum, a sufficient amount of spectroscopically pure and thoroughly dry oxygen is permitted to enter the system, and hence the cell, to establish a pressure of approximately milli- 'meter of mercury, and concurrently therewith the oven |01 is moved into .place over the manifold IIH to bring the temperature up to approximately 260 degrees centigrade. at 'which temperature it is maintained for about 15 'minutes. At this point, the oven may be removed and the cell permitted to cool to room temperature while still leaving the cell subjected to the oxygen pressure; but, if a cell of higher resistance and sensitivity is desired, the cell should be evacuated concurrently with the removal of the oven. l

The oxygen may be admitted to the cell in the following manner: A flask `||2 of spectroscopically pure oxygen has an elongated neck Il! telescoped over the sealed off tip I H of the flask. Adjacent to the upper portion of the neck H3, a mercury chamber H5 is formed by sealing into the neck of the flask a partition IIB having an upwardly extending tubulature to the end of which a porcelain disk H8 of fine -porosity is fused. Adjustably supported above the disk Ill is a similar disk IIS made of the same material and fused to the end of a glass arm |20 connected through a bend |2| with the exhaust system, as indicated at |22. The arm |20 is held in the desired position by a clamp |23 montes en a .stand m, the letter also sup- As long-as the disk Ill is held in the position in which it is shown in Fig. 13i. e., spaced substantially from the disk IIB-the mercury in the chamber IIB effectively blocks the flow of gases through either of the disks or H0. It should be understood that this is so even though the tip Ill has been broken off by magnetically rais ing and then dropping a small, glass enclosed. iron armature |26 located within the neck ||3 of the flask, thus allowing oxygen from the flask to enter the neck |I3. The flask ||2 is originally at atmospheric pressure.

By lowering the arm so that the disk H0 comes in contact with the disk H0, the mercury is forced away from the faces of these disks, and oxygen is permitted to ilow slowly into the exhaust system through the tube |20. Y

When oxygen is to be admitted to the manifold |0|, a stopcock |21 is closed, which disconnects the vacuum pump from the manifold, and the tube |20 is then lowered in the chamber ||l to permit a flow of oxygen into the manifold |0I. 'I'he desired pressure may be obtained by means of a Pirani gauge, generally designated |20, which is connected by a tube to the exhaust system, and which includes a meter |20 calibrated to show pressures.

After suillcient oxidation and heat treatment has been effected within the cells attached to the manifold |0|, the stopcock |21 is again opened, it being understood, of course, that the tube |20 through which oxygen is admitted has been raised to theposition shown in Fig. 13 as soon as in any other suitable manner consistent with this l disclosure, the cells are either evacuated and sealed oi! or a suitable inert gas fili, such as helium or argon, is introduced after evacuation of the oxygen.

Another example of how activation may be effected consists in first admitting oxygen to a pressure of 20 microns while the cells are at room temperature and then baking the tubes at approximately 230 degrees centigrade for about l5 minutes with the oxygen kept at that pressure.

f The oven is then removed and the cells allowed to cool to room temperature, after which the oxygen is pumped out and the cells are again heated by the oven to approximately 360 degrees centigrade, the cells being kept under high vacuum during this operation. When the cells have reached a temperature of 360 degrees centigrade, about 360 of a millimeter of oxygen is introduced into the cells for a period of approximately 15 seconds. The oven is then removed and the oxygen pumped out simultaneously. The cells are then thoroughly evacuated and sealed off, or evacuated and gas filled, as before.

There is some evidence that in both of the illustrative procedures for activation given above, a part of the oxygen treatment is strictly a chemical oxidation process and a part is more or less of an absorption of oxygen by the thallous sulfide. This probably takes place in anysuccessful activation of thallous sulfide.

PHYsIcAx. CHARACTERISTICS or THE PHorocILLs The physical characteristics of a photocell produced in accordance with my invention show outstanding advantages over cells which have heretofore been produced using thalious sulilde as the photo-sensitive material. These characteristics include sensitivity, stability. dark resistance, frequency response, noise level. etc., which will hereinafter b e separately considered.

Sensitivity The full line curve |00 in Fig. 18 shows the response of a cell made in accordance with my invention to an equal energy spectrum, and the dotted line curve III in the same figure shows a similar curve for typical prior art thallous sulnde cells.v It will be observed' that the thalious sulfide cells, both of the prior art and of my design, have their peak response at about 1 mu, but the significant thing about the curves shown in Fig. 18 is the fact that my cell has a sensitivity, on the average, which is several times that of prior art cells, as indicated by the difference in the ordinates between the two curves. Actually, I have been able to construct cells in accordance with my method having sensitivities which are many times that of the prior art cells. For example, if one considered the ratio of the dark resistance of a cell to the resistance effected by a V4 foot candle-illumination produced by a tungsten illament lamp, the prior art cells will show a sensitivity based upon this ratio which may have a factor of around 2, whereas yin my cell, computed on the same basis, the factor may average 6 or more. On occasions, considerably higher factors have been obtained.

It should be understood that in the spectral response curve shown in Fig. 18, the abscissa represents wave length, and the ordinates represent the percentage of change in resistance per microwatt of energy.

Stability One of the outstanding characteristics of my cell is its inherent stability. So far as I am aware, prior art cells have always shown a drift in their dark resistance; and, after exposure to light sources of any substantial intensity, such, for example, as ordinary room light, the dark resistance will drop to a relatively low value and gradually drift back over a period of possibly months, if at all, to higher values. By way of contrast, my cell has a stable dark resistance and may be exposed to room light, or even stronger light, and the dark resistance will remain the same, or at least will recover within a period of a very few minutes.

It should be understood that the fatigue effects which appear in the prior art cells are apparently of two kinds: Electrical fatigue and light fatigue. The electrical fatigue is` evidenced by a drop in dark resistance when a voltage is applied to the cell. Light fatigue Vis evidenced by the proy. nounced drop in the dark resistance of the cell after exposure to a light source;

These characteristics of prior art cells and cells made in accordance with my invention are illustrated in Figs. 19 and 20, the former showing changes in dark resistance in a typical prior art cell when subjected to electrical and light fatigue, and the latter showing the reaction of one of my cells to the same fatigue conditions.

Referring iirst to Fig. 19, in which the ordinates represent cell resistance expressed in megohms and the abscissas represent time expressed in minutes, a typical prior art cell may start with a dark resistance of megohms, and, when Qlleted into an external circuit to which an light source, the cell resistance rwill again rise;.

but, even with it disconnected from the external circuitand the source of E. M. F., the cell will ordinarily vnot return to its original dark resistance, and, if it does, it is only after a lapse of a long period of time, possibly months.

This is illustrated in Fig. 19', in which the cell drops from an initial resistance of 100 megohms to, say, megohms ydue to electrical fatigue over a period of four minutes, and at which time the cell is subjected to a light vsource iwhich may drop its resistance to 2 megohms. rI'he scale employed in Fig. 19 does not permit the subsequent drift downward of resistance due to light fatigue to be shown very clearly, but it will be evident from this figure that, when the light source is removed and the E. M. F. still applied, after a period of two minutes the resistance of the cell still remains at its relatively low value of less than 10 megohms. Y

By way of contrast, my cell, -When properly made, shows substantially no drop in dark resistance when connected to an external circuit and source of E. M. F., as indicated by the portion i3! of the vgraph shown in Fig. 20. rIhe cell may havean initial dark resistance of megohms, and, when subjected lto room ,light of 10 foot candles after a periodv of four minutes, as indicated at |33 (it could be after the elapse of any period of time), the resistance of the cell may drop due to the photoelectric effect to a value of 2 megohm's, as indicate-d at the point |34. Continued application of the light source for any period of time, hereA shown as two minutes, does not lower the resistance of the cell below that which was reached due tothe photoelectric effect, which means, in other words, that the cell does not exhibit a fatigue effect due to the light source. Actually, some cells have shown substantially no light fatigue when subjected to light intensities up to 1000 foot candles.

Upon removing the light source, as indicated at |35 and with a direct current E. M. F. still applied, the resistance of the cell under ordinary conditions immediately -rises to its original dark resistance value of 20 megohms, although some cells will iirst rise to a resistance of possibly I8 or I9 megohms and return to the original dark resistance value of 20 'megohms within a period of three or four minutes. This slightly delayed return of some cells to their original dark resistance value is indicated by the dotted line H6 in Fig. 20.

summarized, it may be said that my cell 4is generally characterized with regard to stability by exhibiting neither electrical fatigue nor light fatigue under the conditions heretofore discussed, and in both of these respects the cell has marked. advantages over thallous sulfide cells heretofore known.

Dark resistance When thallous sulfide photocells are' to be used with alternating current amplifiers, it is important for their dark resistance to be low 14- f enough to match properly the input' impedanc of the amplifier. Normally, dark resistance of less than 30 megohms is desirable for this reason, and a dark resistance of less than 5 megohms is even more desirable. Prior'art thallous sulfide cells, due to their method of fabrication and preparation, have had dark resistance values which ordinarily ran between megohms and 500 megohms, but I have been unable to produce photocells by lmy method which have the required sensitivity and still-have a dark resistance within the ranges oi' necessary for proper `use with A. C. ampliiers.l The particular grid which I employ in my cell, as heretofore described, in conjunction with controlled activation of the thallous sulfide are factors in achieving the low dark resistance which characterizes my cell.

Frequency response A photocell made in accordance with my invention has its best response to relatively lowfrequencies on the order of 100 cycles or less, although the cell will respond upto as high as 5,000 cycles. 'The frequency response of the cell may, however, be greatly enhanced by using, instead of a plurality of grid lines, only a pair of such' lines and concentrating the light source on this reduced target area. Preferably, the two grid lines should be separated a distance of approximately 1 millimeter and should not be overlapped moreA than a few millimeters, the exact amount of overlap depending upon the shape of the image to be received.

Noise Vfactor minimum by using appropriate materials and, Y technique for applying the grid to the cell. For

example, I have found that forming my grid of Aquadag, as `hereinbefore described, produces a cell with substantially lower noise level than a cell having a grid ruled with a lead pencil. Gold and platinum gridsy also produce a relatively low noise factor but are not as desirable as Aquadag due to their yunfavorable affinity for the/'photosensitive covering.

By using Aquadag to form the conducting lines of the grid and to connect the grid with the electrodes, I have been able to produce cells which have a noise factor approaching that due to theram able to produce one having a very desirable signal to noise ratio.

Directional response In the form of the invention shown in Fig. 8,

the photo-sensitive element is so located that the cell is responsive to radiations from all sides of the cell. It is only when radiations emanate and travel along the axis of the cell and have no components which strike the cell at an angle to its axis that the cell is unresponsive to such radiations. Obviously, there may be slightly greater responsiveness of the cell when the radiations emanate from a point directly in front ofvor in rear of the cell, as viewed in Figs. 8 and 9, as distinguished from those emanating from the sides thereof due to the fact that greater area is ex- -posed to such radiations; but, even so. the tube will respond to all such radiations. assuming they carry suilicient energy to be effective in the par- VY@cular circuit to which the cell is connected.

Lmzmurv or Ritsrousl: In common` with all photo-resistive cells, re-

sponse of the cell is non-linear with light intensities. However, the cell does not saturate in direct sunlight.

Although the procedures which have been dislclosed herein will ordinarily produce satisfactory cells, there are many variables which will occasionally cause a cell to fail to meet specication.

However, persons skilled in the art with the teaching of this disclosure should be able to pro-- duce ph'otocells which l are substantially better than prior art cells, particularly -with respect to sensitivity. stability, and initial dark resistance:

' and it is this advance over the prior art which I the material is under controlled oxidation conditions.

I claim:

1. In a photo-sensitive cell, a hermetically sealed envelope having a wall capable of transmitting radiations in the infra-red region of the spectrum, a photo-sensitive surface in the exivelope adapted to receive infra-red radiations passing through said wall, said surface including /a deposit of activated thallous sulfide which has been formed Von the surface by evaporating a quantity of chemically pure thallous Smilde in the envelope while under a vacuum and then activating the thallous sulfide which condenseson the surface by admitting limitedlquantities of oxygen to the envelope.

2. A photoconductive cell of the type includin a photo-sensitive element encased'within a hermetically sealed container, said cell being characterized by a dark resistance of less than 50 megoh'ms. high sensitivity to radiations between 8,000 and 13,000 angstroms, and a noise factor approaching that produced by thermal effects alone, said cell including a photo-sensitive surface of activated thallium sulfide formed as a deposit in the cell under controlled oxidation conditions and maintained in its activated state by not thereafter subjecting it to uncontrolled oxidation and a grid on the surface formed of graphite deposited from a. colloidal graphite suspension.

3. A photo-sensitive cell of the type including a photo-sensitive element encased within a hermetically sealed container, said photo-sensitive element including activated thallous sulfide formed as a deposit in the cell under controlled oxidation conditions and maintained in its activated state by not thereafter subjecting it to uncontrolled oxidation, and said cell being characterlzed by a dark resistance of less than 50 megohms and which does not vary appreciably when subjected to an E. M. F. of approximately 50 volts direct current.

4. A photoconductive element for use in a cell 16 p t of the class described comprising a glass plate, a grid consisting of intercalated conducting lines applied to one face of the plate, spaced conductors sealed into opposed portions of lthe plate and v each having an exposed portion, means for electrically connecting the exposed portion of 'one of the conductors ,to the ends of all even numbered lines of the grid and the exposed portion,of the other conductor to the ends of all odd numbered lines of the grid, and activated thallium sulfide on the grid surface of the plate, said activated thallium sulfide being formed as a deposit in the cell under controlled oxidation conditions and maintained in its activated state by not thereafter subjecting it to uncontrolled oxidation, said gridv lines constituting a deposit of graphite from a colloidal graphite suspension.

5. In a photo-sensitive cell, a hermetically sealed glass envelope having Aat oneend a reentrant portion closed by a relatively flat plate normal to the axis of the cell, a photo-sensitive material deposited on the upper surface of said plate and having a grid thereon composed of spaced, parallel.conductinglines, a pair of conductors sealedl through the envelopeand connected to opposed sides of the plate and having their ends exposed on the interior of the envelope, y and conducting means for connecting one side of the grid to the exposed end of one conductor and the other side of the grid to the exposed end of the other conductor.

s. The method of activatlng'thamum sumde which consists in placing a quantity of thallium sulilde in an enclosure under high vacuum, admitting limited quantities of dry oxygen to the enclosure and then heating the enclosure to cause a reaction to take place between the thallium sulde and oxygen at an elevated temperature. and then again reducing the pressure in the enclosure to a high vacuum, and using the thallium sulfide thus activated as a photo-sensitive element without thereafter subjecting it to uncontrolled oxidation.

7. A resistance element composed of thallium and sulphur reacted to form thallous sulilde and activated by combining the thallous sulfide with limited quantities of oxygen in an enclosed vessel under the influence of heat, after which the activated thallous sulfide is' kept in stable form by being maintained out of further contact with oxygen.

` 8. 'I'he method of processing a thallium sulfide photoceil which comprises the steps of rst depositing a quantity of thalloussultlde on a surface within the cell while continually maintaining the cell under high vacuum, and then causing limited oxidation of the thallous sulfide to take place under the influence of heat.

9. The method of processing a thallium sulilde photocell which comprises the steps of first forming a deposit of thallous sultlde on a surface within the cell while continually maintaining the cell under high vacuum, and then causing limited oxidation of the thallous sulfide to take place under the influence of heat.

10. A photo-conductive cell having a photosensitive element composed of activated thallous sulde formed as a deposit in the cell under controlled oxidation conditions and maintained in its activated state by not thereafter subjecting it to uncontrolled oxidation and characterized by having a stable dark resistance when subjected to electrical potentials of approximately 50 volts direct-current.

11. A photo-'conductive cell having a photosensitive element composed of activated thallous sulfide formed as a deposit in the cell under controlled oxidation conditions and maintained in its activated state by not thereafter subjecting it to uncontrolled oxidation and characterized by having a stable dark resistance and a sensitivity factor of or more.

12. A photo-conductive cell having a photosensitive element composed of activated thallous sulfide formed as a deposit in the cell under controlled oxidation conditions and-maintained in its activated state by not'thereafter subjecting it to uncontrolled oxidation and characterized by having substantially no electrical or light fatigue when subjected to electrical potentials of at least volts and light intensities up to l0 foot candles.

13. The method of preparing a. photocell having an envelope enclosing a photo-sensitive element which consists in evacuating the envelope, depositing a thin layer of thallous sulfide on a surface within the envelope by evaporating a quantity thereof in the envelope while under vacuum and causing it to condense upon said surface, then admitting limited quantities of substantially pure oxygen-to the envelope, and heat treating the thallous sulfide deposit in the presence of said oxygen.

14. The method of preparing a photocell having an envelope enclosing a. photo-sensitive ele- .ment which consists in evacuating the envelope,

depositing a thin layer of thallous sulfide on a surface within the envelope by evaporating a quantity thereof in the envelope while under vacuum and causing it to condense upon said surface, then admitting limited quantities of substantially pure oxygen to the envelope, heat treating the thallous sulfide deposit in the presence of said oxygen, and then again evacuating the envelope.

15. The method of preparing a, photocell having an envelope enclosing a photo-sensitive element which consists in evacuating the envelope, forming a deposit of thallous sulfide on a surface within the envelope and admitting to theenvelope limited quantities of oxygen, and then removing all excess oxygen from the envelope.

16. The method of preparing a photocell having an envelope enclosing a photo-sensitive element which consists in evacuating the envelope, depositing thallous sulfide on a surface within the envelope by evaporating a quantity thereof in the envelope while maintaining a vacuum within the envelope, introducing oxygen into the envelope at a. pressure of less than a few millimeters of mercury, subjecting the envelope to heat, and then removing excess oxygen fromvthe envelope.

17. The method of making a photocell having a photo-sensitive element hermetically sealed therein which consists in forming a grid of conducting lines on a surface Within the envelope, then while the envelope is under a vacuum depositing thallous sulfide on the grid by evaporating a quantity thereof within the envelope and causing it to condense upon said grid, admitting limited quantities of oxygen to the envelope, and finally removing excess oxygen and he'metically sealing the envelope.

18. The method of processing a thallium sulfide photocell which comprises the steps of first forming a deposit of thallous sulfide on a surface within the cell while continually maintaining thercell under high vacuum, and then subjecting the thallous sulfide deposit to multiple stages of lirnited oxidation and heat treatment.

19. The method of preparing a photocell having an envelope enclosing a photo-sensitive element which consists in evacuating the envelope, evaporating a, quantity of pure thallium in the envelope while under vacuum and causing it to condense upon a surface within the envelope, permitting a, limited quantity of sulphurgvapors to react with the thallium thus deposited to form thallous sulfide, and then subjecting the thallous sulfide thus formed to limited oxidation and heat treatment.

20. A photo-cell having a photo-conductivev element comprising a grid formed of graphite deposited from a colloidal graphite suspension and a. coating of activated thallous sulfide over the grid, said activated thallous sulfide being lformed as a deposit inthe cell under controlled oxidation conditions and maintained in its activated state by not thereafter subjecting it to uncontrolled oxidation.

21.-'The method of preparing and activating thallous sulphide which comprises evaporatlng sulphur in a vacuum chamber and causing the sulphur vapor to react with melted thallium in the chamber, then separating the thallous sulphide thus formed from unreacted thallium by the difference in the specific gravity of the two substances, placing the thallous sulphide thus formed in an enclosure under high vacuum, admitting limited quantities of dry oxygen to the enclosure, and then heating the enclosure to cause a reaction to take place between the thallou-s sulphide and oxygen at an elevated temperature, and then again reducing the pressure in the enclosure to a high vacuum.

ROBERT J. CASHMAN,

REFERENCES CITED UNITED STATES PATENTS Number Name Date 537,125 Rovensky Apr. 9, 1895 919,078 Ribbe a Apr. 20, 1909 986,558 Farkas Mar. 14, 1911 1,316,350 Case Sept. 16, 1919 1,336,957 Hedenburg 'Apr. 13, 1920 1,697,451 Baird Jan. 1, 1929 1,728,073 Neale Sept. 1,0, 1929 1,807,056 Zworykin May 26, 1931 1,944,194 Sackville Jan. 23, 1934 1,978,165 Metcalf et al Oct. 23, 1934 1,997,479 Burg Apr. 9, 1935 2,034,334 Falkenthal Mar. 17, 1936 2,036,457 Calsow Apr. 7, 1936 FOREIGN PATENTS Number Country Date 407,755 Great Britain Mai. 27, 1934 412,665 Great Britain July 5, 1934 

